The throw() method resumes the execution of a generator by throwing an error into it and returns an object with two properties done and value.




The exception to throw. For debugging purposes, it is useful to make it an instanceof Error.

Return value

An Object with two properties:

  • done (boolean)
    • Has the value true if the iterator is past the end of the iterated sequence. In this case value optionally specifies the return value of the iterator.
    • Has the value false if the iterator was able to produce the next value in the sequence. This is equivalent of not specifying the done property altogether.
  • value - any JavaScript value returned by the iterator. Can be omitted when done is true.


Using throw()

The following example shows a simple generator and an error that is thrown using the throw method. An error can be caught by a try...catch block as usual.

function* gen() {
  while(true) {
    try {
       yield 42;
    } catch(e) {
      console.log("Error caught!");

var g = gen();;
// { value: 42, done: false }
g.throw(new Error("Something went wrong"));
// "Error caught!"
// { value: 42, done: false }


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Generator.prototype.throw' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of 'Generator.prototype.throw' in that specification.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support ? 26 (26) ? ? ?
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? ? 26.0 (26) ? ? ?

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

ECMAScript6 Generator JavaScript Method Prototype Reference